Tip growth and branching the HGU

The hyphal tip grows and when the hyphae has grown for a while it branches. When these branch hyphae have grown for a while more they branch in their turn eventually forming a mycelium. In trying to describe fungal growth this process was studied in detail and it was observed that as the total mycelium length increases the total number of tips also increases. Both these measurements of fungal growth increased exponentially. If you then divided the total hyphal length with the total number of tips in the mycelium a constant was found for each fungus and growth condition. Thus it appeared as if fungal growth could be simply described as an exponential growth of tips with its accompanying length of hyphae, the Hyphal Growth Unit (HGU). Mycelium growth could then be treated as an exponential growth of HGUs like the HGUs had been bacteria. Figure 2 illustrates one of the key experiments leading up to the HGU model for describing fungal growth.

Fig 2. Total hyphal length (open circles), total number of hyphal tips (open squares) and hyphal growth unit length (closed circles) against time of incubation of a colony of Geotrichum candidum. The hyphal growth unit length was calculated by dividing the total hyphal length with the total number of tips.

The problem with the HGU hypothesis is that it seems to apply only when mycelia are very small and in liquid cultures. Note that in Fig 2 the mycelium is microscopic. The total length of the hyphae in the mycelium is less than 10 mm and the number of tips is less than 64. Therefore there has been a lot of work trying to develop models that can describe growth of large, nutrient translocating mycelia growing in heterogenous environments. One rather successful way has been to treat the mycelium as a autocatalytic biomass unit whose biomass growth and spread into the environment is dependent on the nutrients it contacts, uptake of nutrients, translocation of nutrients etc. The mathematics involved is similar to fluid dynamics calculations. Fig 3 shows a model for how a fungus grows out in one dimension over soil from a substrate (a piece of wood) using nutrients and energy from the wood for growth.

Fig 3. The fungal biomass (dotted area) contains basically 3 parameters, m=biomass, l=labelled nutrient, and si=intracellular substrate concentration. The biomass starts on top of the wood (top figure) and then grows out over the soil (bottom) that is assumed to contain no available nutrients for growth for this particular fungus. Growth rate in a particular area is assumed to be proportional to the intracellular substrate concentration, Si. The local Si is dependent on uptake of substrate, Se, from the wood, translocation of substrate from the wood, use of the substrate for growth and respiration.

The strength of these new models is that they can be relatively easy tested and the relative contribution of different processes can be assessed. Using the model in Fig 3 it was predicted that the label distribution of a labelled substrate would look like in Fig 4A if translocation of nutrients inside the mycelium was passive and like in Fig 4B if the translocation was active and energy dependent (proportional to the intracellular substrate concentration). These models could then be compared to a real experiment (Fig 5) and it can be clearly seen that the label distributions at different times better fit with an active translocation of nutrients.

Fig 4. Distribution of a labeled nutrient at different times after start of the experiment. The label is added to the inoculum at time 0 and growth is assumed to be out over a non-substrate. In the top diagram (A) the translocation of label is assumed to be by passive diffusion. The bottom diagram (B) shows the predictions if the translocation of label inside the mycelium is active and dependent on the available energy locally in the mycelium.

Fig 5. Experimentally measured label concentration in a mycelium of Arthrobotrus superba growing out from a piece of wood into soil. The top diagram shows the distribution of 14C labelled non-metabolizable glucose analogue added to the wood in trace amounts. The bottom diagram shows the distribution of 32P added to the wood in trace amounts.

Even more interesting it could be seen that there seemed to be a maximum length a mycelium could reach out into the soil from the piece of wood. This could also be modelled (Fig 6). In this case a the soil had more bacteria and thus more energy was needed for maintenance compared to what is seen in Fig 4 and 5 explaining the shorter length the mycelium grows out. It was modelled how far out from the inoculum it could be predicted the mycelium could reach if the size of the wood block was increased. It is obvious that the fungus would be able to reach further out into the soil the larger the food bases size. The advantage of active translocation would be greater the larger the food base size. Experimental results from food base size 1 and 5 fitted the model with active translocation better.

Fig 6. Maximal mycelium outgrowth over a non-nutrient area from a food base as a function of food base size. The open circles shows the predictions using a model where nutrient translocation is assumed to be passive. The closed circles and the line shows predictions if translocation is assumed to be active and dependent on intracellular nutrient concentration. For food base size less than 3 there is little difference but for larger sizes the maximal outgrowth is much greater for an actively translocating mycelium. Red circles shows the maximum outgrowth measured experimentally for foodbase size 1 and 5 indicating that the fungus was actively translocating nutrients.